1. Field of the Invention
The present invention relates to an electrode for a light-emitting semiconductor device formed on a surface of a p-type GaN-base compound semiconductor, and to a method of producing the electrode.
2. Description of the Prior Art
In recent years, GaN-base compound semiconductor materials are drawing attention as a semiconductor material for light-emitting devices which emit short-wavelength light. The GaN-base compound semiconductor is formed on various oxide substrates such as sapphire single crystal or a III-V Group compound substrate by the metal organic chemical vapor deposition method (MOCVD method), a molecular beam epitaxy method (MBE method) or other such method.
A GaN-base compound semiconductor is a III-V Group compound semiconductor generally represented by AlxGayIn1xe2x88x92xxe2x88x92yN 0xe2x89xa6xxe2x89xa61, 0xe2x89xa6yxe2x89xa61, 0xe2x89xa6x+yxe2x89xa61).
In the case of a light-emitting device formed by laminating layers of this GaN-base compound semiconductor that uses a substrate of an electrically insulating material, such as a sapphire substrate, an electrode cannot be provided on the back surface of the substrate, unlike when a semiconductor substrate such as a GaAs or GaP substrate is used that is electrically conductive. Accordingly, a pair of positive and negative electrodes are formed on the same surface of the light-emitting device. Also, when electricity is passed through the pair of electrodes to produce light emission, as the sapphire or other such substrate material is an insulator, the light is emitted from the surface on which the electrodes are provided. Namely, the light is emitted upward.
A characteristic of the GaN-base compound semiconductor material is that the current diffusion in the transverse direction is small. Due to this characteristic, even when electrodes are formed and light is emitted by passing electricity therebetween, the major part of the current flow takes place directly beneath the electrodes, as a result of which the light emission is limited to the region right under the electrodes and does not readily diffuse to the peripheral region of the electrodes. Therefore, in the case of conventional opaque electrodes, the light emission is interrupted by the electrode itself and cannot be taken out from the upside of the electrode. As a result, the intended improvement in the light emission intensity is not achieved.
To overcome this drawback, JP-A-6-314822 discloses a technique relating to the device structure whereby a light-permeable electrode comprising a very thin metal is used as a p-type electrode and formed almost over the entire front surface of the device to thereby allow the emitted light to pass through the light-permeable electrode and be emitted externally from the upper side. In this disclosure, Au, Ni, Pt, In, Cr, or Ti, for example, is used as the electrode material and the metal film formed by vapor deposition is heat-treated at a temperature of 500xc2x0 C. or higher to induce sublimation of the metal, so that the thickness is reduced to from 0.001 to 1 xcexcm to thereby impart light permeability. The term xe2x80x9clight-permeablexe2x80x9d as used herein with reference to the electrode refers to an electrode through which light emission generated under the electrode can be observed. To enable observation to take place through the electrode, the electrode must have a light transmittance of at least 10%.
However, such a thin metal film has low strength that makes it impossible to directly bond wires to the thin film for injecting electrical current from an outside source. For this reason, electrodes for use in semiconductor light-emitting devices generally employ a structure comprising forming, in addition to the light-permeable electrode, a wire-bonding electrode having electrical contact with the light-permeable electrode, and using this wire-bonding electrode to connect the wire used to carry current to the light-permeable electrode.
When a light-permeable electrode is formed using thin metal film, as shown by FIG. 23, the structure generally used comprises forming the wire-bonding electrode 8 on the light-permeable electrode 7. However, with this structure it is difficult to ensure adhesion between the front surface of the light-permeable electrode 7 and the lower surface of the wire-bonding electrode 8, sometimes causing the wire-bonding electrode 8 to peel off during the electrode production process.
To overcome this, JP-A-7-94782 discloses a technique for improving bonding properties, illustrated by FIG. 24. In this arrangement, a window 70 is formed in the light-permeable electrode 7 via which the surface of the semiconductor 9 is exposed, the wire-bonding electrode 8 is formed on the window 70 to effect direct contact between the wire-bonding electrode 8 and the surface of the semiconductor 9.
In most cases a thick film about 1 xcexcm in thickness is used for the wire-bonding electrode as a way of absorbing the impact of the wire bonder. Because it is that thick, light permeability cannot be imparted to the wire-bonding electrode. This means that light emission occurring directly below the wire-bonding electrode is interrupted by the wire-bonding electrode, and therefore cannot be emitted to the outside. Thus, to achieve higher emission brightness, a structure is required whereby current is not injected into the semiconductor portion directly beneath the wire-bonding electrode, but flows instead to the light-permeable electrode.
JP-A-8-250768 discloses a technique whereby current does not flow to the region below the wire-bonding electrode. This is achieved by providing the semiconductor layers below the wire-bonding electrode with a high-resistance region by various methods such as by forming a silicon oxide layer, leaving a region that is not subjected to p-type formation treatment, using annealing or ion implantation and so forth. The high-resistance region prevents current flowing under the wire-bonding electrode, directing the current instead to the light-permeable electrode to thereby efficiently use the current.
However, in the disclosure of JP-A-8-250768, the structure providing the high-resistance region under the wire-bonding electrode requires the formation of silicon oxide layers and steps to increase the resistance of the semiconductor. Thus, the process is complicated and production takes long time. For example, in order to form silicon oxide layers, it is necessary to use photolithography to effect patterning, or plasma CVD processes and the like. Similarly, photolithography, ion implantation, annealing and other such processes have to be used to form a high-resistance semiconductor region. All these processes are complex time-consuming.
Also, when the above-described high-resistance region arrangement is to be applied to the configuration of the above JP-A-7-94782 in which the wire-bonding electrode 8 is provided on the window 70 (FIG. 24), the high-resistance region is formed in the semiconductor 9 beneath the wire-bonding electrode 8. This produces an arrangement in which the current has to flow from the peripheral portion 8a of the wire-bonding electrode 8 into the semiconductor 9, via the light-permeable electrode 7, generating light emission in the injection region 91. Since the peripheral portion 8a acts as a barrier to the generated light, the light emission cannot be taken out upward. The light emission is therefore wasted, reducing emission efficiency.
The object of the present invention is to provide an electrode for light-emitting semiconductor devices, that uses a simple structure that is able to securely block current flow under the wire-bonding electrode and can improve the light emission efficiency.
The present invention attains the above object by providing an electrode for a light-emitting semiconductor device, formed on the surface of a p-type GaN-base compound semiconductor, comprising a light-permeable electrode formed to come into contact with the surface of the semiconductor, and a wire-bonding electrode that is in electrical contact with the light-permeable electrode and is formed to come into partial contact with the surface of the semiconductor with at least a region in contact with the semiconductor having a higher contact resistance per unit area with respect to the semiconductor than a region of the light-permeable electrode in contact with the semiconductor.
The wire-bonding electrode may have a multilayer structure in which the topmost layer is formed of Al or Au.
The light-permeable electrode may comprise a first layer formed to come into contact with the surface of the semiconductor and comprising at least one member selected from the group consisting of Au, Pt and Pd, and a second layer formed on the first layer and comprising a light-permeable metal oxide containing an oxide of at least one metal selected from the group consisting of Ni, Ti, Sn, Cr, Co, Zn, Cu, Mg and In.
The second layer has an oxygen composition that gradually decreases from the second layer toward the first layer in the region near the interface between the second layer and the first layer.
The first layer may contain a metal element which is a main component of the metal oxide constituting the second layer.
The light-permeable electrode may be formed to overlay the upper surface of the wire-bonding electrode at a portion at which the wire-bonding electrode is disposed.
The light-permeable electrode may be formed to overlay the periphery of the upper surface of the wire-bonding electrode.
The electrode light-permeable electrode may be formed to cover the entire upper surface of the wire-bonding electrode.
A portion of the second layer of the light-permeable electrode that overlays the wire-bonding electrode may be removed to expose the first layer.
The electrode for a light-emitting semiconductor device according to the present invention also includes an electrode formed on the surface of a p-type GaN-base compound semiconductor, comprising a light-permeable electrode formed to come into contact with the surface of the semiconductor, and a wire-bonding electrode that is in electrical contact with the light-permeable electrode and is formed with a bottom surface in partial contact with the surface of the semiconductor and an upper surface overlaid by the light-permeable electrode.
The light-permeable electrode may be formed to overlay the periphery of the upper surface of the wire-bonding electrode.
The light-permeable electrode may be formed to cover the entire upper surface of the wire-bonding electrode.
The present invention also provides a method of producing an electrode for a light-emitting semiconductor device, formed on a surface of a p-type GaN-base compound semiconductor, comprising a first step of forming a wire-bonding electrode on a portion of the surface of the semiconductor, a second step of forming a first layer on the surface of the semiconductor, the first layer comprising at least one member selected from the group consisting of Au, Pt and Pd and being formed to overlay the upper surface of the wire-bonding electrode at a portion at which the wire-bonding electrode is located, a third step of forming on the first layer a second layer that comprises at least one metal selected from the group consisting of Ni, Ti, Sn, Cr, Co, Zn, Cu, Mg and In, and a fourth step of forming a light-permeable electrode by heat-treating the first and second layers in an atmosphere that contains oxygen.
The method of producing an electrode for a light-emitting semiconductor device according to the present invention may instead comprise a first step of forming a wire-bonding electrode on a portion of the surface of the semiconductor, a second step of forming an alloy layer on the surface of the semiconductor, the alloy layer comprising an alloy that contains at least one metal selected from the group consisting of Au, Pt and Pd and at least one metal selected from the group consisting of Ni, Ti, Sn, Cr, Co, Zn, Cu, Mg and In and being formed to overlay the upper surface of the wire-bonding electrode at a portion at which the wire-bonding electrode is located, and a third step of forming a light-permeable electrode by heat-treating the alloy layer in an atmosphere containing oxygen to form on the semiconductor side a first layer comprised of metal or alloy, and a second layer comprised of a light-permeable metal oxide formed on the first layer.
As described in the foregoing, the region of the wire-bonding electrode in contact with the semiconductor is formed to have a higher contact resistance per unit area with respect to the semiconductor than the region of the light-permeable electrode in contact with the semiconductor, making it possible to securely prevent current flowing under the wire-bonding electrode, so that all the current from around the wire-bonding electrode is injected into the light-permeable electrode, from where it enters the laminate body and contributes to the light emission function. That is, light emission is not generated under the wire-bonding electrode, so that with the light not being obstructed by the wire-bonding electrode, substantially all the light that is generated can be emitted upward from the light-permeable electrode. Thus, the current can be effectively utilized and the light emission efficiency improved.
This electrode configuration having a wire-bonding electrode and a light-permeable electrode can be formed by growing thin films using a method such as a vapor deposition method. The process is very simple, involving just the vapor deposition of the metal material, so formation of the films can be effected rapidly. That is, current flow under the wire-bonding electrode can be securely blocked by means of a simple structure that can be readily formed without having to undertake complex processes.
Further features of the invention, its nature and various advantages will be more apparent from the accompanying drawings and following detailed description of the invention.